A strategy for expanding hematopoietic stem cells and early lineage- committed progenitors has been described. The oncogenic mutant of the H- ras gene which can induce proliferation in hematopoietic cells has been cloned into a vector containing the regulatory elements of the TN10 operon. The gene can be temporally regulated in a reversible manner. Hence the targeted cells can be expanded in the presence of an inducer (tetracycline). once a significant increase in the number of cells is attained, the inducer is removed in order that the amplified cells can revert to a normal phenotype. Amplification procedures have been outlined that will allow the characterization of cells which proliferate in response to activated ras. The cells can than be evaluated for their ability to reconstitute SCID mice. It is hoped that reconstitution of humans can then be contemplated, once a further fail-safe mechanism that mediates excision of the inserted gene is introduced. The cells thus amplified will also be available for basic research directed at determining additional means of regulating proliferation with the purpose of further expansion of progenitor and lineage precursors.